Antenna



y 1942. N. E. LINDENBLAD 2,284,434

ANTENNA Filed Feb. 24, 1941 '2 Sheets-SheetN 1 INV EN TOR.

- N/LS E. Ll DENBLAD BY ATTORNEY.

May 26,1942. N. E. LINDENBLAD ANTENNA Filed Feb. 24, 1941 2 Sheets -Sheet 2 Fly. 3

FREQUENCY IN M. C

Emax.

min.

INVENTOR. N/LS E. L/NDENBLAD vwmw ATTORNEY.

Patented May 26, 1942 ANTENNA Nils E. Lindenblad, Port Jefierson, N. Y., assignor to Radio Corporation of tion of Delaware America, a corpora- Application February 24, 1941, Serial NO. 380,147

Y Claims.

The present invention relates to antennas and, more particularly, to short wave antennas.

A primary object of the present invention is to enable the transmission of a wide band of radio waves with low frequency discrimination over .said band.

A further object of the present invention is to provide an antenna suitable for use on ultrahigh frequencies which has a broad frequency response curve.

Another object of the present invention is the provision of a mechanically simple antenna structure which is metallically grounded at all times to provide protection against lightning.

Still a further object of the present invention is the provision of an antenna particularly suitable for the radiation of vertically polarized waves and which can be simply and conveniently combined in arrays in an axial direction to provide a substantial degree of directivity in the plane of polarization. The simplest and preferred embodiment will thus appear in antennas for vertical polarization.

These and other objects of the present invention which may appear from the following detailed description and'claims are attained by the provision of an antenna in the form of somewhat tapered half wave long conductive tubes supported vertically above a grounded conductive surface at a distance therefrom of a half the operating wavelength for the lowest element. Considering an antenna consisting of only one such unit, this tube or shell is energized from a transmitter or generator of high frequency oscillations of any desired type through the medium of a coaxial transmission line having its inner conductor connected to the interior surface of the radiating tube at such point that the effective impedance of the antenna seen from the transmission-line is substantially equal to the impedance of the transmission line. In order to insure as perfact matching as possible the connection is tapered from the interior surface of the tube to the transmission line to transfer energy from the transmission line to the radiating tube with a minimum of variation incharacteristic impedance. thus, without reflection. The supporting column of the radiating tube is of substantial diameter so that it may contain the transmission line and a compensating stub connected thereto at a'distance of a quarter wavelength from the point of connection to theradiating tube. The compensating stub serves to broaden the frequency response curve of the antenna.

The invention will be more completely understood by reference to the following detailed description which is accompanied by drawings in which Figure 1 illustrates in vertical cross-section an embodiment of the invention; Figure 2 illustrates a horizontal cross-section of the antenna, while Figure 3 is a curve illustrating the frequency characteristics of the antenna shown in Figure 1, and Figures 4, 5 and 6 are vector diagrams explanatory of the principles of the invention.

In Figure 1 of the drawings reference numeral I denotes the tapered conductive tube which serves as a radiator of the high frequency oscillations supplied thereto by transmission line TL. The radiating tube I is supported by a hollow conductive mast 2 over which it is supported in a coaxial arrangement. The radiating tube I is electrically connected to the supporting mast 2 at the upperend, Theupper end is closed by a covered plate or roof 3 which also closes the top of the hollow supporting mast. The radiating tube I is maintained radially in alignment with the axis of the supporting mast 2 by means ofa metallic ring 4 which is situated substantially at the midpoint of the radiating tube I. The conductive ring 4, in addition to maintaining the alignment between the tapered radiating tube I and the mast 2 also serves to make the inside electric length of the radiating'tube I equal to approximately a quarter wave which results in a maximum impedance between the lower end of the radiating tube and the supporting mast 2. The antenna structure is supported above a conductive sheet 5 at a distance therefrom of approximately a half wave operating wavelength. The conductive sheet 5 may conveniently be the roof of a tall building or tower. The transmission line 'I'L which energizes the antenna is shown as being of the concentric or coaxial cable type having an inner conductor 6 and an outer shell I. The inner conductor "6 is connected to the inner surface of the radiating tube I by a connecting plate 8.

As will be seen by reference to Figure 2, the connecting plate 8-is so formed that a smoothly tapering connection is established between the interior of the radiating tube I and the inner conductor 6 of the transmission line 'I'L. This gradual change in dimensions of the conductor prevents any abrupt changes in the characteristic impedance of the system which would restrict the band width of the antenna and transmission line assembly by causing reflection of energy back into the transmission line.

While the antenna as thus far described has a comparatively broad response characteristic, as shown by curve I0 of Figure 3 wherein it is indicated that a band width of 5.7% is obtained at a midband frequency of 128 magacycles with less than l /2% reflection, this may be considered as'insuflicient for television purposes. Some additional compensation may therefore be supplied. This is accomplished by means of compensation stub I! connected to the conductors of transmission line TL at a distance from the point of connection of the transmission line to the radiating tube I of a quarter of the operating wave length. The compensation stub 12 comprises a section of concentric transmission line having an inner conductor "3' and an outer shell FL The inner conductor and outer shell are connected together at a distance from the point of connec-- tion to transmission line T1. of a quarter of the operating wavelength. The compensating stub has such dimensions as to obtain an .midband frequency it provides parallel resonance at this frequency and its presence has very little effect on the wave distribution along the transmission line TL. At frequencies on either side of the midband frequency, however, the antenna presents a reactive component to the transmission line TL. The compensating stub l2 then presents an equal reactive component. Due to the fact that these two reactive components are spaced along the transmission line a distance of a quarter wavelength at rnidband frequency they also compensate very effectively for one another at frequencies adjacent to the middle of the band and the band width of the antenna system is greatly improved.

By means of the vector diagrams in Figures 4, 5 and 6 it will now be demonstrated how compensation is obtained from such an arrange ment.

The voltage and the current of a traveling wave along a transmission line are in phase. When, however, looking in a certain direction along a line, the energy or power flowing in one direction must naturally be considered as of positive value while the flow in the opposite direction is of negative value. Hence, while the power in one direction is expressed by the equation W=EI it must be expressed by the equation W=EI in the other direction when E and I represent voltages and current respectively. If the expression for the power is written as EI cos 0 and the angle 0 is considered as zero for one direction it must be 180 degrees for the opposite direction. With this concept in mind it is now easy to proceed with the vector geometric presentation. 7

If a line is matched and then a reactance is introduced, either in shunt or in series, a reflection will occur at this point. Between the power source and the point of reflection there will then be two traveling waves of opposite direction.

The difference between the power flowing in the two directions then represents the power of the flow beyond the point of reflection. The voltage-current and powerrelation of a traveling wave is controlled'by the characteristic impedance of the line. Thus,

beyond the point of reflection, and En, the voltage of the direct wave and ER, the voltage of the reflected wave. This expression may also obviously be written E0 ED E122 This may be factored into 0 rl- ER) D R) Since the quantity (En-l-ER) is equal to the maximum voltage on the transmission line and the quantity (ED-ER) is equal to the minimum voltage along the line the voltage of the traveling wave beyond the point of reflection may be expressed by the following equation:

E0 Emuzx min This equation is probably more illustrative if written as follows:

I EL1 EP E0 min,

This shows clearly that the voltage of a traveling wave beyond the point of reflection is the geometric mean of the maximum and minimum voltages on the portion of the transmission line which has reflection thereon. This may be geometrically interpreted as shown in Figure 3.

From the triangular proportionality of the half circle diagram in Figure 4 it may be seen immediately that this construction is the geometric representation of a geometric mean. Since the amplitude of the voltage maximum along the transmission line is the sum of the amplitudes of the voltages of the direct and reflected wave and the minimum is the difference, the geometric relationship between these quantities may be represented as shown in Figure 5. Herein the sum of the minimum and maximum voltages are represented by the diameter of the outer circle diagram while the difference is represented by the diameter of the inner circle diagram. The voltage of the traveling wave beyond the point of reflection is indicated by the vertical vector E0. The direct and reflected voltages are represented by. the vectors ED and ER extending from the center of the two concentric circles.

It has been shown that if the direction of travel of the direct wave is considered positive it voltage and current will be in phase, whereas the voltage and current of the reflected wave must be in phase opposition. Thus we obtain the vector relation as shown in Figure 6 for the parallel reactance case. The voltage and current of the direct wave are indicated as being in phaseby the vertical vector labeled ED, In. The reflected voltage ER is indicated by the circle generated by vector Em with the current vector 1R1 in a phase opposing relationship thereto. It will be seen that there are two conditions for which E0 is a tangent to the circle as indicated by the vectors E01 and E02. One of these is for the case of a parallel capacitive reactance and the other for the case of a parallel inductive reactance.

Since the angular displacement between the vector ED of the direct wave and the vectors Em and Eaz of the reflected waves represents twice it will appearas an inductance of equal reactive value. Therefore, a reflection caused by a reactive transmission on a line can be'compensated by placing an equal reactance in the line at a distance therefrom corresponding to a line angle equal to It may readily be seen from the geometric construction in Figure 6 that the line angle varies with reflection but will approach, 90 degrees or a quarter wavelength if the reflection is not excessive.

By compensating in the foregoing manner, with a reactance stub, a characteristic as shown by curve 20 in Figure 3 may be obtained. It will be seen herein that a 16% band Width at a midband frequency of 120 megacycles is obtained with less than 7' reflection. This represents a very ample band width for television purposes, particularly if the results are translated to equivalent structures in the 50 megacycle region.

While I have particularly shown and described a particular embodiment of my invention, it is to be distinctly understood that my invention is not limited thereto but that improvements and modifications within the scope of the invention may be made.

I claim:

1. A short wave antenna comprising a vertical half wave radiator in the form of a hollow truncated cone closed at the small end, said radiator being supported above a planeconductive surface at a distance therefrom equal to a half wavelength at the operating frequency by a hollow conductive mast, said mast extending within the large end of saidcone and connected to said cone at the small end thereof, a conductive connection between said mast and the interior of said radiator at substantially its midpoint, means for energizing said radiator from a source of high frequency energy comprising a concentric transmission line having an inner conductor and an outer sheath within said hollow mast, the outer sheath of said transmission line being connected to said mast, means for connecting said inner conductor to the inner surface of said radiator at such point that the impedance of said radiator presented to said transmission line is substantially equal to the impedance of said transmission line, said connecting means being in the form of a flat plate ta pered in width and having its small end connected to said inner conductor and its large end connected to said inner surface and means for compensating for the reactance of said antenna comprising a similar reactance connected in shunt to said transmission line at a distance from its point of connection to said radiator of a quarter wavelength.

2. In combination with a hollow short wave radiator which has at least one circular section,

means for coupling a conductor of the transmission line to said radiator including a conductive plate within said radiator, said plate lying in a plane parallel to said section and connected along one edge with the interior of said radiator, the remaining edges of said plate being so formed that the impedance of said radiator is transformed to a value equal to that of said transmission line.

3. In combination with a hollow short wave radiator which has at least one circular section means for coupling a conductor of the transmission line to said radiator including a conductive plate within said radiator, said plate lying in a plane parallel to said section and connected along one edge with the interior of said radiator, the remaining edges of said plate being so shaped as to form a gradual tapering connection between said transmission line and said radiator.

4. A short wave antenna comprising a half wave radiator supported normal to a plane conductive surface at a distance therefrom equal to a half wavelength by a conductive support, means for energizing said radiator from a source of high frequency energy comprising a transmission line having a pair of conductors, one conductor of said transmission line being con-' nected to said radiator at such point that the impedance of said radiator presented to said transmission line is substantially equal to the impedance of said transmission line by a tapered conductive connection, said connection means being in theform of a flat plate tapered in Width and lying in a plane normal to the axis of said radiator and having its small end connected to said one conductor and its large end connected to said radiator and means for compensating for the reactance of said antenna comprising a similar reactance connected in shunt to said transmission 'line at a distance from its point of connection to said radiator of a quarter wavelength.

5. A short wave antenna comprising a vertical radiator in the form of a hallow truncated cone, said radiator being supported above a plane conductive surface at a distance therefrom equal to a half wavelength at the operating frequency by a hollow conductive mast, said mast extending within the large end of said cone and connected to said cone at the small end thereof, a conduce tive connection between said mast and the interior of said radiator at substantially its mid point, means for energizing 'said radiator from a source of high frequency energy comprising a concentric transmission line having an .inner conductor and an outer sheath within said hollow mast, the outer sheath of said transmisison line being connected to said mast and the inner conductor being connected to the inner surface of said radiator at such point that the impedance of said radiator presented to said transmission line is substantially equal to the impedance of said transmission line and means for cornpensating for the reactance of said antenna comprising a similar reactance connected in shunt to said transmission line at a distance from its point of connection to said radiator of a quarter wavelength, said similar reactance comprising a quarter wave section of concentric transmission line short-circuited at one end and connected at its other end to said transmission line.

6. A short wave antenna comprising a half wave radiator in the form of a hollow truncated cone closed at the small end, said radiator being frequency energy comprising a concentric trans-V mission line having an inner conductor and an outer sheath, the outer sheath of said transmission line being connected to said' support and tapered conductiv means for connecting the inner conductor to the inner surface of said radiator, said conductive means having its small endconnected to said inner conductor and its large end connected to said radiatorrand means for compensating for the. reactance of said antenna comprising a similar reactance connected in shunt'to said transmission line at a distance from its point of connection to said radiatorof a quarter wavelength, said similar reactance comprising a section of concentric transmission line short-circuited at a distance. from itsconnection to said transmission line equal to a quarter wavelength.

'7. A short wave antenna comprising a radiator in the form of a truncated cone, said radiator being supported normal to a plane conductive surface at a distance therefrom equal to a'half wavelength at the operating frequency conductive support, said support extending within said cone and connected to its interior, means for energizing said radiator from a sourceof high frequency energy comprising a concentric transmission line having an inner-conductor and an outer sheath, the outer sheath of said transmis sion line being connected to said support and the inner conductor being connected to said radiator at such point that the impedance-of said radiator presented to said transmission line is substantially equal to the impedance of saidtransmission line and means for compensating for the reactance of said antenna comprising a section of concentric transmission line connected at one end-in shunt to said transmissionline at a distance from its point of connection to said radiator of a quarter wavelength, said section being short-circuited at a distance from its connection to said transmission line equal to a quarter wavelength.

8. A broadband antenna comprising "a vertical radiator in the form of a hollow truncated cone closed at the small end having a length equal to a half wavelength at the mid frequency ofsaid band, said radiator being supported. above a plane conductive surface by a hollow conductive mast, said mast extending within the large end of said cone and connected to said cone at the small end thereof, a conductive connection between said mast and the interior of said radiator at substantially its midpoint, means for energizing said radiator from a source of high frequency energy comprising a concentric transmission line having an inner conductor and an outer sheath within said hollow mast, the outer sheath of said transmission line being connected to said mast, means connecting said inner conductor to the inner surface of said radiator at such point that the impedance of said radiator presented to said transmission'line is substantially equal to the impedance of said transmission line, said connectingmeans being in the form of a flat plate tapered in width and having its small end con-' nected to said inner conductor and its large end connected to said radiator and means for compensating for the reactance of said antenna at frequencies removed-fromsaid midband com-' prising a similar reactance connected in shunt to said transmission line at a distance from its point of connection to said radiator of a quarter wavelength at said midband frequency.

9. A broadband antennarcomprising a radiator having a cross-section ,which is a substantial fraction of any wavelength within said broadband, having a length equal to a halfwavelength at the mid frequency of said band; said radiator being supported normal to a plane conductive surface at a di'stance'therefrom equal to the length of said radiator by a conductive. support; said support extending within one end of said radiator and connected thereto at substantially its midpoint, means for energizing said radiator from a source of high frequency energy comprising a transmission line having a pair of conductors, one conductor of said transmission line being'connected to said support, means for connecting the other of said conductors to said radiator at such point that the impedance of said radiator presented to said transmission line is substantially equal to the impedance of said transmission line, said connecting means being in the form of a flat plate tapered in width and having its small end connected to said one con ductor and its large end connected -to said radi ator and means for compensating for the reac- 1 tance of said antenna at frequencies removed from said midband comprising a similar reactance connected in shunt to said transmission lineat a distance from its point of connection to said radiator of a quarter wavelength at said midband frequency.

110. A broadband antenna comprising a radiator having a cross-section which is a substantial fraction of any wavelength within said broadband, having a length equal to a half wavelength at the mid frequency of said band, said radiatorbeing supported normal to a plane conductive surface at a distance therefrom equal to the length of said radiator by a conductive support, said support extending within one end of said radiator and connected thereto at substantially its midpoint, means for 'energizing'said radiator from a source of high frequency energy comprising a transmission line having a pair of conductors, one conductor of said transmission line being connected to said support, means for connecting the other of said conductors to said radiator at such point that the impedance of said radiator presented to said transmission line is substantially equal to the impedance of said transmission line, said connecting means being in the form of a flat plate tapered in width and having its small end connected to said one conductor and its large end connected to said radiator and means for compensating for the reac tance of said antenna at frequencies removed from said midband comprising a section of transmission line similar to the first mentioned transmission line having one end connected in shunt to said first line at a distance from its point of connection to said radiator equal to an odd multiple, including unity, of a quarter wavelength at said midband frequencyyand short-circuited at'such distance from its connection to said first line that the reactance of said section is equal to the reactance of said radiator at frequencies removed from said midband.

NILS E. LINDENBLAD. 

